Seamless metal tubes can be produced by successive plastic deformation of a starting billet. First, the billet is heated in a furnace to a temperature of about 1220-1280° C. Then the billet is pierced longitudinally to obtain a pierced semi-finished article with a thick wall and a length 1.5 to 4 times longer than that of the starting billet. Next, a mandrel is introduced into this semi-finished article. This semi-finished article is then passed through a rolling mill (referred to below as “main rolling mill”) that can gradually thin the wall by means of suitable diameter-reducing operations and increase the length of the finished product. As is well known, the rolling mill comprises a plurality of rolling units. Each unit comprises a stand on which rolls with profiled grooves are mounted. Usually, a system comprises three profiled rolls and the profiles of the grooves of the three rolls, all connected together, define the outer profile of the tube produced by the rolling unit.
As mentioned above, the main rolling mill requires the arrangement of a mandrel inside the tube being processed to counter the radial thrust exerted by the rollers during rolling. In order to exert this counter action, the mandrel must be extremely stiff in the radial direction. Moreover, in order to ensure a high-quality finish for the inner surface of the tube, the mandrel must have an outer surface which is as smooth as possible. Because of this requirement, it would be extremely difficult to manufacture mandrels consisting of several parts joined together. The joining zone is in fact necessarily characterized by an irregular surface. Moreover, this zone would be too delicate to withstand the radial rolling pressure.
Use of a retained mandrel, where the mandrel is both axially constrained and retained so as to advance at a controlled speed, is well known. This solution has a notable drawback. The single section of the mandrel, while being constrained, is advanced axially along the rolling mill and is thus engaged in full deformation conditions in successive rolling stations. Inside the rolling stations, the mandrel is subjected to high thermal and mechanical stresses due to the deformation energy and the friction produced by the sliding contact of the tube material. The passage through more than one rolling station therefore causes a significant increase in the mandrel temperature, thereby resulting in the need to provide several mandrels which are identical to each other. The multiple mandrels allow each one of them to be suitably cooled at the end of rolling and then lubricated for the next rolling cycle. In addition to this, it must be considered that the individual mandrel must be made entirely of a particularly high-quality material in order to withstand the stresses typically arising during rolling. From the above, it is clear that a considerable outlay is required for the mandrel stock in order to be able to ensure operation of the main rolling mill.
Downstream of the main rolling mill, the tube is extracted from the mandrel and the final finishing operations are performed so as to obtain a tube which is able to comply with suitable quality control standards. The main parameters which must be verified are the wall thickness and the outer diameter of the tube. At present two different types of plant which are able to perform the final finishing operations are known.
A first type of plant envisages an extracting mill, downstream to the main rolling mill and in series, capable of extracting the semi-finished tube from the mandrel. This extracting mill usually comprises three stands.
During the subsequent processing operations, it is no longer possible to directly modify the thickness of the tube wall. It is therefore advisable, in this type of plant, to carry out a control of the wall thickness soon after the extractor. In this way, if the semi-finished tube has a wall thickness which is different from the desired thickness it is possible to perform automatic adjustment of the main rolling mill so as to correct the thickness along the following tube sections.
A sizing mill is positioned, off-line, downstream to the extractor and the thickness control point. This sizing mill comprises a plurality of fixed stands (usually 10-12) which are able to define the final diameter of the tube so that it complies with the required standard. In order to obtain a good result in terms of the diameter, it is advisable to ensure a uniform temperature for the tube inside a suitable furnace so that uniform contraction of the tube is also achieved during subsequent cooling. During this processing step, the tube exiting from the main rolling mill may have different temperatures along the various sections, depending on the geometric conditions of the tube and transient factors during the process. Therefore, the furnace which precedes the sizing mill must have dimensions that allow the entire tube to be housed internally so that the tube may have a uniform temperature of about 950° C.
Following the action of the sizing mill, the final diameter of the tube is brought into compliance with the desired standard. The wall thickness, however, may fail to comply with the standard because sizing mill modifies the thickness of the wall in an uncontrollable and sometimes unpredictable manner. Downstream to the sizing mill, a station for controlling the final thickness of the tube may also be provided and may, if necessary, correct the thickness of the semi-finished article upstream, within the main rolling mill. It is clear, however, that this control operation is performed at a later stage and that the conditions which caused a deviation of the thickness from the required standard may have, in the meantime, changed again, thereby invalidating the effectiveness of the control operation.
This first type of plant, although widely used, is not without drawbacks. Firstly, the furnace arranged between the extracting mill and the sizing mill represents an additional outlay and, since it must remain constantly in operation, generates high running costs. Moreover, from a logistical point of view, the fixed-roll sizing mill requires a large mandrel stock in order to be able to adapt to the different diameters required, different steels used and their characteristics. Finally, as mentioned above, a control of the final thickness of the tube wall is performed indirectly and is thus unable to ensure small tolerance values.
A second type of known plant envisages the arrangement, extracting/sizing mill downstream and in series to the main rolling mill. This extracting/sizing mill comprises a plurality of adjustable-roll stands and is thus able to extract the tube from the mandrel and control the final tube diameter. A control of the wall thickness is performed just after the extracting/sizing mill. In this way, if the finished tube has a wall thickness which is different from the desired thickness, it is possible to perform automatic adjustment of the main rolling mill so as to correct the thickness along the following tube sections.
Although this type of plant is clearly more compact than the plant described previously, there are a number of drawbacks which make use of the plant not particularly advantageous. The extracting/sizing mill comprises many adjustable stands (10-12), making it very complex and expensive. Moreover, accurate control of the tube diameter cannot be performed on-line. It should be remembered that, at the end of the rolling process, the tube moves along the plant at a speed of about 5-6 m/s. It is therefore very difficult to implement feedback control which allows checking of the tube parameters and real-time modification of the rolling mills. This difficulty is increased when there are variations in temperature along the tube. These temperature variations cannot be effectively compensated for and can result in corresponding variations in the final diameter of the tube.
The object of the present invention is therefore to overcome, at least partly, the drawbacks mentioned in the above prior art references. In particular, a task of the present invention is to provide a continuous rolling plant which allows more effective control over both the outer diameter and the wall thickness of the finished tube. Moreover, a task of the present invention is to provide a continuous rolling plant which requires a smaller initial outlay and lower running cost. Finally, a task of the present invention is to provide a continuous rolling plant which allows simpler management from a logistical point of view.